In order to be able to demodulate received signals which have been transmitted via a radio channel, the radiofrequency received signals are usually first of all downconverted to a lower frequency. The further signal processing then takes place at this intermediate frequency. In this case, the received signals may be downconverted either to the frequency zero or to an intermediate frequency other than zero. After the signals have been downconverted, it must be ensured that only those signal components which lie within the frequency band defined by the channel are fed to the further processing stages. For this purpose, a channel filter is provided, which suppresses the undesirable frequency components. Said channel filter may be a bandpass filter, and in particular a complex bandpass filter. In the case of a complex bandpass filter, image frequency components which would lie within the passband range are also suppressed. In particular, the channel filter may be designed as a polyphase filter.
It has been found that channel filters provided for the processing of signals with a relatively high intermediate frequency can be integrated on the chip only with difficulty. Moreover, complying with the tolerances required for the signal processing on the chip poses problems in the case of receiver architectures with a relatively high intermediate frequency. Therefore, a transition has been made to the use of so-called low-IF architectures (IF: Intermediate Frequency), in which the received signal is downconverted to a relatively low-frequency intermediate frequency approximately corresponding to the frequency spacing between the individual channels. The implementation of the channel filters on the chip does not pose any problems when using low-frequency carrier frequencies. The required tolerances can also be complied with here. However, problems arise during demodulation due to the low carrier frequency of the downconverted signal, which lies in a range in which the signal spectrum has a still significant component.
The demodulator for a low-IF architecture has hitherto usually been embodied as a quadricorrelator. In a quadricorrelator, the in-phase and quadrature signals coming from the mixer are fed to a filter whose limiting frequency approximately corresponds to the carrier frequency of the in-phase and quadrature signals fed. This filter, which may be designed as a polyphase filter, for example, causes a frequency-dependent phase shift of the input signals. Afterward, the phase-shifted signals occurring at the output of the filter are cross-correlated with the in-phase and quadrature signals present at the input of the filter. This yields a low-frequency signal whose amplitude is a measure of the phase shift effected by the filter. The frequency modulation of the input signal can be tracked on the basis of this low-frequency signal. This signal can subsequently be evaluated further by means of a decision unit.
However, superposed on this useful signal is an undesirable, higher-frequency component which, although it can be suppressed in part by summation of the demodulated in-phase and quadrature signals, nevertheless appreciably disturbs the further evaluation. Particularly in the case of nonlinearities and mismatches in the circuit, this disturbance signal may have considerable amplitude. Owing to the low carrier frequency in low-IF architectures, the frequency of the disturbance signal is near to the spectrum of the data signal. A subsequent filtering which could mask out this disturbance signal becomes very difficult as a result. A further disadvantage when using quadricorrelators is the relatively high outlay on circuitry which is necessary to implement them.